The invention relates to a driving apparatus and a driving method of an actuator which is used in an optical disk apparatus, a compact disk apparatus, or the like and, more particularly, to a driving apparatus and a driving method using a pulse width modulation for forming a pulse signal of a duty ratio according to instruction data and driving a load.
Recently, although the optical disk apparatus, compact disk apparatus, and the like have been developed, various serve mechanisms are used in those apparatuses. For example, the optical disk apparatus using a rewritable magnetooptic medium includes: a focusing servo mechanism for allowing a laser beam to trace a surface oscillation of the disk medium; a tracking servo mechanism for correcting an eccentricity and allowing the laser beam to trace a guide groove for recording and reproducing; a carriage control mechanism for seeking an optical head to a recording and reproducing track of the disk medium; an external magnetic field control mechanism for rotating and moving a permanent magnet to give an external bias magnetic field upon erasing or recording of data or switching a polarity of an electromagnet; further, a loading/ejecting control mechanism for loading and ejecting a recording medium enclosed in a cartridge; and the like.
As such an actuator of the servo mechanism, an inductive load such as a motor, a solenoid coil, or the like is used. For example, by supplying a current to the solenoid coil, a mechanical motion such as linear movement, rotational movement, or the like is obtained. To drive the actuators for the motor, the solenoid coil, and the like, a power amplifying circuit using a power amplifier is used. The power amplifying circuit can be classified into a voltage driving type, a current driving type, or a pulse width modulation (PWM) driving type.
FIG. 1 shows a power amplifying circuit of the conventional voltage driving type. A non-inverting amplifier 206 using an operational amplifier, an inverting amplifier 208, resistors R1 and R2 are provided for a power amplifying circuit 204 of the voltage driving type. Output terminals of the amplifiers are connected to both ends of an actuator coil 210. Current instruction data from a servo circuit section 200 is converted to a control voltage by a D/A converter 202. The control voltage is inputted to the non-inverting amplifier 206 and the inverting amplifier 208. When the input voltage has a plus polarity when it is seen from a potential at a center point, a drive current shown by a solid line 212 is supplied from the non-inverting amplifier 206 to the inverting amplifier 208 through the actuator coil 210, thereby driving the actuator in the forward direction. When the input voltage has a minus polarity as it is seen from the potential at the center point, a drive current shown by a broken line 214 is supplied from the inverting amplifier 208 to the non-inverting amplifier 206 through the actuator coil 210, thereby driving the actuator in the reverse direction.
FIG. 2 shows a power amplifying circuit of the conventional current driving type. An inverting amplifier 218 using an operational amplifier, an inverting amplifier 220, and resistors R3 to R9 are provided for a power amplifying circuit 216 of the current driving type. Output terminals of the amplifiers are connected to both ends of the actuator coil 210. The resistor R9 is a current detection resistor of small resistance value that detects a current flowing in the actuator coil 210. A detection voltage of a load current by the resistor R9 is fed back to the inverting amplifier 218 and to the inverting amplifier 220. The load current is a constant current controlled so that the detection voltage coincides with the conversion voltage from the D/A converter 202. Thus, a constant current corresponding to current instruction data of the servo circuit section 200 is obtained.
FIG. 3 shows a power amplifying circuit of the conventional pulse width modulation driving type. A pulse width modulating (PWM) circuit 222 is used in combination with a bridge type driver 224. Current instruction data from the servo circuit section 200 is inputted to the PWM circuit 222. The PWM circuit 222 generates a pulse signal having a duty ratio according to the current instruction voltage. Specifically speaking, the PWM circuit 222 generates a forward direction PWM signal E101, a reverse direction PWM signal E102, and an enable signal E100. The bridge type driver 224 bridge couples FETs 228, 230, 232, and 234. The actuator coil 210 is connected between the FETs 228 and 232 and the FETs 230 and 234. When the forward direction PWM signal E101 is supplied in a state in which the enable signal E100 is valid, the FETs 228 and 234 are on/off controlled and a current in the forward direction shown by the solid line 212 flows in the actuator coil 210. When the reverse direction PWM signal E102 is supplied in a state in which the enable signal E100 is valid, the FETs 230 and 232 are on/off controlled and a current in the reverse direction shown by the broken line 214 flows in the actuator coil 210.
However, in the power amplifying circuit of the voltage driving type of FIG. 1, although the circuit construction is simple, a resistance value of a coil load changes in accordance with a temperature change. There is, consequently, a fear that the load current fluctuates and that the servo gain changes and also that a servo deviation or an oscillation of a servo system occurs. Similarly, there is also a problem such that even for fluctuation of power source voltage, the load current changes and the servo gain changes. According to the power amplifying circuit of the current driving type of FIG. 2, although a high precision is obtained by constant current control even when there are temperature change and power source fluctuation, circuit construction is complicated by only an amount of circuits for detecting the load current and feedback controlling. Particularly, in the case where a number of servo mechanisms are provided, the whole circuit scale increases.
Further, according to the power amplifying circuit of the pulse width modulation driving type of FIG. 3, electric power consumption is reduced owing to a duty control using an accumulating function of energy which the inductive coil load has, so that high efficiency is obtained. However, since the pulse width modulation driving type is fundamentally a voltage driving type, the servo gain fluctuates due to fluctuation of the load current by the resistance change of the coil load depending on the temperature and fluctuation of the load current in association with fluctuation in the power source voltage, so that servo deviation, oscillation of the servo system, or the like occurs.
A circuit of the PWM driving type having a function that is substantially equivalent to the current driving type for detecting a load current and for feedback controlling the load current by adjusting a duty ratio is also considered. However, since an A/D converter, an analog comparator, and the like for feedback controlling the current are necessary in addition to the pulse width modulating circuit, circuit construction is complicated. Particularly, in the case where a number of servo mechanisms are provided, the whole circuit scale increases.
Further, in the optical disk apparatus or the like, in order to control the number of servo mechanisms, digital signal processors (DSP) have recently been used. According to the digital signal processor, an analog signal which was detected by a sensor or the like and inputted from an outside is converted to a digital signal and supplied to the processor. Servo control data (current instruction data) is formed by an arithmetic operating process by a program control, and the servo control data is converted to an analog signal or pulse width modulation signal, thereby driving a coil load. Such a process can be also executed by a high speed microprocessor in place of a digital signal processor (DSP) 236. In the subsequent description, it is assumed that a high speed microprocessor can also be used in place of the digital signal processor 236.
As shown in FIG. 4, hitherto, A/D converters 244 and D/A converters 248 and, further, PWM output circuits 246 for generating pulse width modulation signals are ordinarily provided for the digital signal processor 236 as standard specifications in addition to a CPU 238, an ROM 240, and an RAM 242. Among them, the area which is occupied in an LSI chip by the digital signal processor of the PWM output circuit 246 is small and cost performance of the circuit is high. Therefore, it can be said that a construction in which the bridge type driver 224 is combined with the digital signal processor 236 installed with the PWM output circuit as a standard specification and is used for a servo control of the optical disk apparatus is an effective form from viewpoints of the simplification of the circuit, decrease in costs, improvement of the precision, and the like. However, the PWM output circuit 246 installed as a standard specification in the DSP 236 is also fundamentally the voltage driving type. Servo gain fluctuations due to fluctuations in load current (from either a resistance change in the coil load, which depends on temperature, or fluctuations in load current in association with fluctuations in the power source voltage) result in servo deviation, an oscillation of the servo system, or the like. Therefore, a method of providing a function that is substantially equivalent to the current driving type for detecting load current and for feedback controlling the load current by adjusting the duty ratio is also considered. However, an A/D converter, an analog comparator, and the like are necessary and the circuit is complicated.
Therefore, it is difficult to build all of the circuits including a driver into the LSI chip of the digital signal processor 236. A discrete circuit is also necessary between the DSP 236 and the bridge type driver 224, and there is a problem such that an advantage that will be obtained by using the digital signal processor cannot be sufficiently effected.